Magnetic structures in integrated circuits and hard-disk read-heads include multi-layer devices comprising ferromagnetic films, conductive films and insulation films. These layered magnetic structures include magnetic tunneling junctions, spin-valve transistors and pseudo spin valves.
Most commonly, physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes are used for deposition of these films. There are problems associated with these techniques; pinholes, film thickness non-uniformity and impurities at the film interfaces have caused devices to fail. For example, PVD results in three-dimensional growth of cobalt into islands on aluminum oxide, rather than two-dimensional film growth. In addition, ferromagnetic metals are rather sensitive to corrosion and thus require very careful treatment.
The problem of thickness non-uniformity can be mitigated using a chemical-mechanical polishing technique (CMP). A CMP technique for making a magnetic structure has been described in patent application WO 00/38191, published Jun. 29, 2000. Unfortunately, CMP causes oxidation of some ferromagnetic materials. The oxygen must be removed and the film restored to elemental metal. This reduction reaction should be effected at a low temperature (e.g., less than about 300° C.), as high temperatures can destroy the functionality of the device.
Magnetic random access memories (MRAMs) have many desirable properties. The magnetic polarity of the soft magnetic layer can be switched very quickly, in as little as a nanosecond. The MRAM cells can be packed close together; they can be scaled down to densities used for state-of-the-art DRAMs (dynamic random access memories). MRAM fabrication requires fewer mask steps than DRAM fabrication, thus simplifying production and saving time and costs. In addition, the MRAM is non-volatile. Unlike the DRAM, it is not necessary to supply the MRAM with continuous or periodic power. Once data has been written to the MRAM cell it will remain until it is rewritten and needs no additional power. Thus, it is expected that MRAMs have the potential to replace DRAMs, static RAMs (SRAM) and flash memory in a wide range of applications, such as cell phones, MP3 players, personal digital assistants (PDAs) and portable computers. The manufacturing of magnetic central processing units (MCPU) will also be feasible. MCPUs can be reprogrammed on the fly to match any specific task. Before this can happen, however, the remaining manufacturing problems of MRAM structures must be solved.
One form of a basic MRAM cell comprises a single current-sensing element and a three-layer magnetically functional sandwich. These cells are written to and read via current passing through adjacent conducting lines. In the sandwich a very thin, insulating or “tunnel dielectric” layer separates two magnetic layers. One of the magnetic layers is “soft,” which means that relatively small magnetic fields can change the magnetic polarity of the material, i.e., the material has low coercivity. The other magnetic layer is “hard,” which means that the polarity of the material changes only under the influence of a relatively large magnetic field, i.e., the layer has high coercivity. The magnetic fields associated with writing and reading currents in the conducting lines cannot change the magnetic polarity of the “hard” magnetic layer. Data is written to the soft magnetic layer of the sandwich by passing a current through two conductor lines that are electrically connected to the magnetic layers.
A single bit of data can be read from the sandwich by using an address line connected to one of the conductor lines. The address line can turn on a sense transistor, depending on the current level that tunnels through the magnetic sandwich. The level of the tunneling current depends on the polarity of the magnetic layers in the sandwich. When the polarity of the magnetic layers is parallel, higher current tunnels through the sandwich than when the polarity of the magnetic layers is antiparallel.
In the MRAM cell, the thickness of the insulating layer in the sandwich is in the nanometer range. The strength of the tunneling current through the insulator is very sensitive to the thickness of the insulator. For example, aluminum oxide insulating thin films may be just four atomic layers thick. Changing the thickness of the insulator by only a tenth of a nanometer may change the tunneling current by an order of magnitude. In addition, the insulator must have high dielectric strength to withstand the operating voltages and provide sufficient tunneling current.
MRAM structures are also sensitive to pinholes in the layers. These can short-out the magnetic memory cell, rendering the device non-functional. The MRAM layers are also temperature sensitive; defects can be expected if the layers are exposed to excessively high temperatures.
The tunneling dielectric can be made, for example, by plasma oxidizing an aluminum metal layer into aluminum oxide. Low surface diffusion of atoms prevents formation of islands and pinholes.
Spin-valve transistors are used as magnetic field sensors. Read-heads for hard-disks and MRAMs can comprise spin-valve transistors.
In a typical spin-valve transistor structure (Si collector/Co/Cu/Co/Pt/Si emitter), Co and Cu layers are deposited by sputtering. The optimum thickness of the Cu layer is about 2.0–2.4 nm. Improved flatness of the Co/Cu interface increases the magnetoresistance effect, which is desirable. The scattering probability of the hot electrons in the base is altered by a magnetic field.